Patent Document 1 (prior document) discloses a device for changing the gear ratio of a transmission for a vehicle power unit by means of a control unit for operating an electric motor as a shift motor (hereinafter referred to as a variable-speed drive device).
The variable-speed drive device disclosed in the prior document is provided with an electric motor, a reduction gear mechanism for reducing the rotational speed of the electric motor, a shift spindle driven in rotation by the reduction gear mechanism, a shift arm (master arm) provided on the shift spindle, and a shift drum rotated intermittently by the shift arm. The shift arm is formed with a guide hole (opening), and a guide pin (stopper pin) extends through the guide hole. The rotational angles of the shift arm in two rotational directions are regulated or determined by abutment of the outer surface of the guide pin with the defining edge of the guide hole.